The site, magnitude, and mechanisms of acute airway responses to interventions, both aerosol and non-aerosol, designed to either dilate or constrict airways of normal subjects, both smokers and non-smokers, and patients with either chronic bronchitis or asthma will be studied in order to better understand the spectrum of airway responses and its relationship to clinical characteristics, potential therapeutic applications, and prognosis in the obstructive syndromes. Predominant location of airways obstruction appears to be detectable by measuring maximal expiratory flow rates with gases of varying density and by assessing the effect of volume history on the magnitude of change in airflow rates and on the degree of density dependence. Determination of both site and magnitude of acute responses should be important in assessing similarities and differences between bronchoconstrictor challenges in normal subjects and non-challenge assessments of patients should relate to the severity and to therapeutic potential in obstructive airway disease. The possibility of specific treatment depending upon clinical state and precipitating factors will be explored in the obstructive syndromes.